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Reversible Self-Assembly of Metal ChalcogenideMetal Oxide Nanostructures Based on Pearson Hardness.

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Communications
DOI: 10.1002/anie.201000774
Surface Functionalization
Reversible Self-Assembly of Metal Chalcogenide/Metal Oxide
Nanostructures Based on Pearson Hardness**
Jugal Kishore Sahoo, Muhammad Nawaz Tahir, Aswani Yella, Thomas D. Schladt,
Enrico Mugnaoli, Ute Kolb, and Wolfgang Tremel*
Dedicated to Professor Bernd Harbrecht on the occasion of his 60th birthday
Nanotechnology has reached a stage of development where
not individual nanoparticles but rather systems of greater
complexity are the focus of concern.[1] These complex
structures incorporate two or more types of materials, an
example of which is the formation of metal–semiconductor
hybrids, which effectively combine the properties of both
materials.[2] The assembly of multicomponent nanoparticles
from constituents with different optical, electrical, magnetic,
and chemical properties can lead to novel functionalities that
are independent of the individual components and may be
tailored to fit a specific application. These applications
include such far-reaching challenges as solar energy conversion,[3] biological sensors,[4] mechanical and optical devices,[5]
and potential methods for drug delivery and medical diagnostics.[6]
A specific challenge is to assemble nanoparticles into a
hierarchical structure. Nanotubes (NT-MQ2)[7] and fullerenes
(IF-MQ2)[8] of layered metal chalcogenides are the purely
inorganic analogues of carbon fullerenes and nanotubes, and
exhibit analogous mechanical[9] and electronic properties.[10]
They consist of metal atoms sandwiched between two inert
chalcogenide layers. Their physical properties[11] are related to
their crystal structures, which contain MQ2 slabs with metal
atoms sandwiched between two inert chalcogen layers. These
MQ2 layers are stacked with only van der Waals contacts
between them. The steric shielding of the metal atoms by the
[*] J. K. Sahoo, Dr. M. N. Tahir, Dr. A. Yella, T. D. Schladt,
Prof. Dr. W. Tremel
Institut fr Anorganische Chemie und Analytische Chemie der
Johannes Gutenberg Universitt
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax: (+ 49) 6131-39-25605
E-mail: tremel@uni-mainz.de
Dr. E. Mugnaoli, Dr. U. Kolb
Institut fr Physikalische Chemie
Johannes Gutenberg Universitt
Welderweg 11, 55099 Mainz (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) within the priority program 1165 “Nanotubes and Nanowires: From Controlled Synthesis to Function” (research program:
“From single molecules to nanoscopically structured materials”).
A.Y. is a recipient of a fellowship from POLYMAT, the Graduate
School of Excellence of the State of Rhineland-Palatinate. T.D.S. is a
recipient of a Carl Zeiss Fellowship. We acknowledge support for the
Electron Microscopy Center in Mainz (EZMZ) from the Center for
Complex Matter (COMATT).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000774.
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chalcogen surface layers from nucleophilic attack by oxygen
or organic ligands makes chalcogenide nanoparticles highly
inert and notoriously difficult to functionalize.
Some progress has been made by employing chalcophilic
transition metals in combination with multidentate surface
ligands: The 3d metals “wet” the sulfur surface of the
chalcogenide nanoparticles whilst the multidentate surface
ligands partially block one hemisphere of the metal coordination environment. This steric shielding prevents an aggregation of the chalcogenide nanoparticles through interparticle
cross-linking.[12]
The assembly of aggregates from different types of
nanoparticles typically relies on chemical modifications of
the nanoparticle surface to achieve a specific linkage. A
bifunctional organic linker molecule having specific anchor
groups for each type of nanoparticle is bound with one of its
anchor groups to the first type of the pre-synthesized nanoparticles. In a subsequent step, the second anchor group is
used for the attachment of the second type of nanoparticles.[13]
The goal is to attach a controlled number of target
molecules while avoiding aggregation through nonspecific
interactions with surfaces and other particles in solution. To
achieve that goal, the nanoparticles have to be stabilized with
a protecting layer containing some chemical anchor points for
further modification. This covalent chemical attachment
offers high stability in different solvents and ionic environments. Therefore, current strategies for the functionalization
of nanoparticles rely on either 1) non-covalent physisorption
of linker molecules to the surface of the nanoparticles,[14]
2) electrostatic anchoring of an additional polymeric layer,[15]
or 3) the use of short bifunctional cross-linkers. These
processes lead to low yields[16] or low surface coverage.[17]
An alternative strategy is to grow nanoparticles directly
on the nanotubes by using colloidal nanoparticle synthesis
methods.[18] Colloidal nanoparticles may have an affinity
based on their acid–base properties, functional groups, or
Pearson hardness[19] for nanotube surfaces that allows their
attachment without the aid of linkers.
Herein we present a novel synthetic strategy based on
Pearsons HSAB (hard/soft acid–base) principle.[19c] that
allows the formation of a hierarchical assembly of metal
chalcogenide/metal oxide nanostructures. The metal oxide
particles can be functionalized in a subsequent reaction step
at room temperature to tailor the chalcogenide surfaces or to
reversibly detach them from the chalcogenide surfaces with
excess surface ligand (Scheme 1). The recycled chalcogenide
2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Chemie
Scheme 1. Representation of reversible immobilization of MnO NPs
on NT-WS2 and surface functionalization with fluorophore-labeled
dopamine.
Figure 1. TEM images of the as-synthesized a) MnO NPs and b) NTWS2. c–e) The attachment of MnO NPs onto NT-WS2. c) TEM overview
image, d,e) HRTEM image of the interface of MnO NPs and sidewalls (d) or the tip (e) of a WS2 nanotube. f) EDX spectrum of the
MnO@NT-WS2 nanocomposite.
Scheme 2. The reversible functionalization of MnO NPs on NT-WS2.
nanoparticles can be used time and again without the use of
organic ligands (Scheme 2).
According to Pearsons HSAB principle, a hard Lewis
acid has a tendency to bind with a hard Lewis base, and a soft
acid with a soft base.[19c] Thus, in the layered metal chalcogenides the soft sulfur surface layer has a tendency to bind
with other nanoparticles containing soft transition metal
cations. The components chosen to build these magnetic
nanohybrids, MnO nanoparticles (MnO NPs) and WS2 nanotubes (NT-WS2), were prepared separately in the first step.
Figure 1 a,b show representative transmission electron microscopy (TEM) images of these building blocks, which were
synthesized according to published procedures.[20] As the
MnO NPs carry oleic acid molecules as surface ligands, they
are soluble in most organic solvents (cyclohexane, toluene, or
THF).[20a] The NT-WS2 were prepared by the sulfidization of
WO3 nanorods obtained by hydrothermal synthesis.[20b] A HR
(high-resolution) TEM image of one such NT-WS2 is shown in
Figure 1 b. The interlayer spacing between the tubular walls
(0.65 nm) is consistent with the (002) d spacing of the 2H-WS2
lattice.
The assembly of oxide/chalcogenide nanostructures was
produced by mixing dispersions of the chalcogenide nanoparticles and metal oxide nanoparticles in toluene by
mechanical shaking. During this process, the metal oxide
Angew. Chem. Int. Ed. 2010, 49, 7578 –7582
nanoparticles were assembled onto the chalcogenide nanoparticle surface by ligand exchange. In this process, the oleic
acid capping ligands on the surface of the oxide nanoparticles
are substituted by the surface sulfur atoms of the chalcogenide nanoparticles. The binding of MnO NPs to NT-WS2 is
illustrated in the TEM images in Figure 1 c–e along with
energy-dispersive X-ray analysis (EDX; Figure 1 f). The soft
basic character of the chalcogenide surface is of prime
importance for the surface binding. The chemisorption of
ions or molecules involves their acidic or basic properties,
which have to be opposite to those of the active surface sites.
The overview TEM image (Figure 1 c) shows that almost all
the nanotubes are covered with MnO nanoparticles, and a
HRTEM image shows that almost all nanoparticles are sitting
on the basal plane of nanotubes. A scanning electron
microscopy (SEM) image (Supporting Information, Figure S2) also gives an overview to confirm the immobilization
of MnO NPs on NT-WS2. EDX of the composite nanotubes
(Figure 1 f) indicates the presence of the elements W, S, Mn,
and O. The analytical data indicate a significant amount of
MnO NPs present on the surface of the NT-WS2, in
accordance with the results of the TEM study.
As described earlier, the tailoring of the chalcogenide
surfaces is difficult and requires some suitable approaches,
whereas the metal oxide surfaces are easy to functionalize.
The MnO NPs surface-bound to NT-WS2 therefore allow the
selective functionalization. In this process, the oleic acid
capping ligands are replaced by free catechol ligands.
Figure 2 a shows the UV/Vis spectrum of the MnO@NTWS2 nanocomposites (black line) and also MnO@NT-WS2
modified with dopamine bound 7-nitrobenzofurazan (NBD)
2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
Figure 2. a) UV/Vis absorption spectrum of as-synthesized WS2 NPs
(black line) and MnO@NT-WS2 nanocomposites functionalized with
dopamine-NBD (gray line/*). b) Confocal laser scanning microscopy
images of NT-WS2 coated with NBD-functionalized MnO NPs. Inset: a
single nanotube. A 40 oil immersion objective (NA 1.25) was used
for the imaging.
dye (gray line/*). A characteristic broad absorption band of
WS2 can be observed at about 660 nm, and the absorption
band due to NBD appears at 490 nm, along with a pronounced maximum at 280 nm, which is probably due to
excitation of the phenyl ring of the dopamine. The 660 nm
absorption band of functionalized NT-WS2 is weakly visible in
the MnO@NT-WS2 composite. The surface decoration of NTWS2 with NBD-functionalized MnO NPs was further demonstrated using confocal laser scanning microscopy (CLSM;
Figure 2 b). A 10 mL droplet of the sample in THF was placed
and dispersed carefully on a thin glass slide, and the solvent
was then evaporated. The fluorescence of the NBD dye was
excited at 514 nm and detected from 520–540 nm. The
overview image of the MnO@NT-WS2 nanocomposite asfunctionalized with NBD ligands (Figure 2 b) shows anisotropic fluorescence images of nearly isolated nanoparticles.
From the fluorescence images, it is reasonable to conclude
that the nanotubes are fully coated with surface-functionalized MnO NPs covalently tethered to the NT-WS2 surface. It
is difficult to comment on the actual size of the functionalized
nanotubes because they are below the resolution limits of
CLSM. In a control experiment, no fluorescence was
observed by exciting the unfunctionalized MnO NPs or
unfunctionalized NT-WS2.
Pearsons HSAB principle states that because of both
their thermodynamic and kinetic properties, hard acids prefer
to coordinate with hard bases and soft acids with soft bases.
The effect of solvation of ions must however be taken into
consideration to gain a proper understanding of the HSAB
principle. Due to their chelating properties, catechol-type
ligands can compete successfully with sulfur for 3d surface
metal atoms. As solvation increases with an increase in
temperature, we used a temperature of 60 8C to remove all
surface-bound MnO NPs from the NT-WS2 surface in the
presence of dopamine as chelating ligand for the metal oxide
nanoparticle surfaces. However, without addition of dopamine, it is not possible to reversibly recover the metal
chalcogenide nanotubes.
Another reason for the stability of the catecholate surface
complexes is valence taumerism of redox-active ligands and
transition metal ions. Catecholate ligands form unstrained
and unsaturated five-membered-ring chelate systems with
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surface metal atoms via negatively charged oxygen atoms.
Complexes with non-innocent electroactive ligands may
exhibit a reversible intramolecular electron transfer between
the metal ions and the ligand, leading to a stabilization
through internal charge redistribution (Supporting Information, Scheme S2).[21]
Scheme 2 illustrates the attachment and detachment of
MnO NPs to NT-WS2. Whereas oxides of soft and borderline
metals are chemisorbed easily to the sulfur surface atoms,
oxides of the hard metals have a much lower tendency for
binding. In contrast, the catechol-type ligands bind efficiently
to hard or borderline metals (such as Ti4+, Fe3+, Al3+, Mn2+).
As a result, the binding of chalcogenide nanoparticles and
catechol-type ligands to surface metal atoms of borderline
metal oxides is a dynamic equilibrium reaction, the position of
which depends on the reactants and temperature. By increasing the reaction temperature to 60 8C, catechol is a preferred
surface ligand for the metal oxide particles. It displaces the
surface sulfur atoms of NT-WS2, leaving unfunctionalized NTWS2 nanotubes behind. Adding fresh metal oxide nanoparticles to NT-WS2 leads to a partial replacement of the
oleate surface ligands by the surface sulfur atoms of NT-WS2
and a concomitant binding of metal oxide nanoparticles to
NT-WS2. This cycle can be repeated several times. The
recycled chalcogenide nanoparticles can then be re-used
(Scheme 3).
Scheme 3. Reversible attachment of MnO Nps on NT-WS2 and surface
modification with fluorophore-labeled dopamine.
The HSAB model has its basis in arguments related to
bonding strengths. It is applied to systems where kinetic
control, entropy of adduct formation, solvation effects
(enthalpic and entropic), ion-pairing effects (enthalpic and
entropic), or lattice energy effects (enthalpic and entropic)
are large and even dominant. When HSAB considerations
are employed, it is implied that the soft–soft (covalent) or
hard–hard (ionic) interactions dominate the chemistry; that
is, that the reactions are either orbital- or charge-controlled.[22]
2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 7578 –7582
Angewandte
Chemie
In summary, we have used the principles of coordination
chemistry to achieve the reversible functionalization of highly
inert chalcogenide nanotubes (NT-WS2) with metal oxide
nanoparticles (MnO NPs). The modification strategy is based
on the chalcophilic affinity of Mn2+, as described by the
Pearson HSAB concept. The surface-bound nanoparticles are
still amenable to functionalization with anchor ligands, such
dopamine. As the chelating dopamine ligand is a much more
potent ligand for surface 3d metals than the sulfur atoms of
the chalcogenide nanoparticles, the MnO NPs can be
detached from the chalcogenide surface at a slightly elevated
temperature. The remaining chalcogenide particles can be the
functionalized again with fresh metal oxide nanoparticles.
The self-assembled hybrid architecture can incorporate
various different selective nanoparticle–substrate interactions
based on well-known surface processes, and it may be
generalized for various layered chalcogenide nanoparticles
and transition metal and main group oxides. This assembly
technique also offers benefits for low-cost and low-waste
manufacturing; such methods are becoming increasingly
important in the development of green nanofabrication
strategies.
The functionalization of WS2 nanotubes opens several
new fields for this class of materials, which have been pursued
actively during the past few years for the related carbon
nanotubes and various oxide materials: 1) the functionalization of chalcogenide nanotubes for the attachment of
electronically active components (metal and semiconductor
nanoparticles, light-harvesting ligands for solar-cell applications) to the sidewalls of the tubes; 2) dispersion of nanotubes, for example, for the integration in composites, which is
of interest because of their exceptional mechanical properties; and furthermore, it allows 3) the fabrication of thin films
by surface binding of chalcogenide particles to oxide surfaces,
which might allow their use as lubricants on seemingly
incompatible ceramic materials.
Received: February 8, 2010
Revised: April 26, 2010
Published online: August 16, 2010
.
Keywords: chalcogenides · HSAB principle · MnO nanoparticles ·
surface functionalization · WS2 nanotubes
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